This fixes a problem seen by a number of people running UML on newer hostkernels. init would hang with an infinite segfault loop.

It turns out that the host kernel was providing a AT_SYSINFO_EHDR of0xffffe000, which faked UML into believing that the host VDSO page could bereused. However, AT_SYSINFO pointed into the middle of the address space, andwas unmapped as a result. Because UML was providing AT_SYSINFO_EHDR andAT_SYSINFO to its own processes, these would branch to nowhere when trying touse the VDSO.

The fix is to also check the location of AT_SYSINFO when deciding whether touse the host's VDSO.